A wavelength conversion device is known to obtain ultraviolet light from continuous visible laser radiation of a fundamental wavelength by wavelength conversion using a nonlinear optical material having optical anisotropy, such as a uniaxial crystal. This nonlinear optical effect is a series of phenomena based on nonlinear polarization induced in the nonlinear optical material. Using the second-order nonlinear optical effect from among them, wavelength conversion (generation of a second harmonic, added frequency, differential frequency, and so on) from laser light can be done. For example, it is usual to perform the wavelength conversion of light such as second harmonic generation (SHG) with the nonlinear optical material, such as a beta barium borate crystal (BBO). Thus, the simplest example of the wavelength conversion is second harmonic generation, and the frequency of the converted light is twice that of incident light and, hence, the wavelength of the converted light becomes half. Simply describing the wavelength conversion of the second harmonic generation as an example, the added frequency generation of two wavelengths and the differential frequency generation of the two wavelengths are also similar to this example when incident light is composed of the two wavelength components. In addition, since most nonlinear optical materials are of crystalline form, one of them is referred to as a crystal hereafter.
Since an amplitude of induced nonlinear polarization in the secondary nonlinear optical effect is proportional to the square of an amplitude of an electric field of incident light, converted light power is proportional to the square of input light power, but the proportional constant is fairly small. Thus, generally, light power converted with small conversion efficiency is low.
Thus, it is arranged to externally provide a resonator for the laser device so that the laser beam light incident on a crystal (hereinafter called "excitation light") is enhanced in its power to emit light with twice the frequency or second harmonic (hereinafter called "SHG light") from the crystal.
However, as mentioned above, it is necessary to tune in a resonance frequency of a resonator with a frequency of laser light entering the resonator so as to efficiently generate SHG light from a crystal using the resonator. This requires feedback control to detect an error of a resonance frequency to suppress the frequency shift of laser light entering the resonator and to tune in a light frequency through the changing of the resonator length of the resonator.
Hence, a conventional wavelength conversion device 100 comprises an electrooptical modulator (hereafter "EO modulator") 108, having an electrooptical effect, disposed between a laser device 102 and a resonator 106 as shown in FIG. 13. Laser light L0 radiated from the laser device 102 enters the resonator 106 through the EO modulator 108. On an extended optical path of laser light L1 radiated from the EO modulator 108, an optical detector 112 connected to an electric circuit 110 is disposed for servo control to drive the EO modulator 108. The electric circuit 110 for servo control is to provide the electric field so as to cause the electrooptical effect at the EO modulator 108.
The resonator 106 comprises an incident mirror 114, sampling mirror 116, first mirror 122, and second mirror 124. Laser light L1 entering the resonator 106 arrives at the sampling mirror 116 through the incident mirror 114 on optical path L1a. The sampling mirror 116 radiates light partially sampled from incident light to the optical detector 112, and guides residual light through reflection to optical path L1b. Light reflected with the sampling mirror 116 arrives at the first mirror 122. The first mirror 122 guides the light to optical path L1c through total reflection. On optical path L1c, the crystal 104 is disposed, and the light reflected with the first mirror 122 arrives to the second mirror 124 through the crystal 104. Thus, the crystal 104 radiates SHG light converted in accordance with incident light with light not converted. The second mirror 124 reflects light not converted at the crystal 104 out of the incident light and transmits converted SHG light L2. Light reflected with the second mirror 124 is guided to optical path L1d and arrives at the incident mirror 114.
At the incident mirror 114, an incident light axis in parallel to optical path L1d and a reflection light axis in parallel to optical path L1a are adjusted, and the incident mirror 114 guides light proceeding on optical path L1d through reflection to optical path L1a.
The wavelength conversion device 100 in such a configuration detects an error of a resonance frequency, i.e., difference between the light frequency and the cavity resonance frequency from a detection signal of an optical detector 112. To detect the frequency difference, the wavelength conversion device 100 needs to modulate laser light L0 through frequency modulation (and phase modulation) of the EO modulator 108 at a high frequency. This is referred to as the FM sideband method, as described by Michio Oka et al., Jpn. J. Appl. Phys., Vol. 31, 1992, page 531; and R. Drever et al., Appl. Phys, B, Vol. 31, 1983, page 97.
However, error detection in a conventional wavelength conversion device 100 has the following problems:
1. Loss of incident power arises due to the surface reflection and internal absorption of the EO modulator. Because of this, the power of incident visible light is lowered, and the final output of SHG light is also lowered.
2. Because of the optical aberration in the EO modulator, wavefronts of laser light radiated from the EO modulator are perturbed and, hence, the coupling efficiency of laser light to the resonator is lowered and therefore SHG light output is also lowered.
3. Since the EO modulator is disposed between a laser device and the resonator, the entire system becomes large.
4. Since the configuration of the EO modulator is necessary, its cost becomes expensive.
In addition, in the conventional wavelength conversion device 100, installing the first mirror 122 on a piezoelectric element (an element to convert electrical signals to displacement), feedback control to tune in a frequency makes the resonator length change through making the first mirror 122 displaced and makes the resonance frequency change.
However, since the first mirror 122 has inertia mass, the response characteristic to a high frequency is lowered. Moreover, the response to the frequency fluctuation of incident light at more than several kHz is difficult because of physical limitations of the piezoelectric element. Therefore, when the frequency fluctuates faster than the mirror displacement can be made, the power of SHG light, that is, the output light, fluctuates or synchronization becomes impossible due to the failure of servo control. These are problems in the conventional wavelength conversion device 100.
Taking into consideration the above facts, the object of the present invention is to obtain, in a small and easy-to-use configuration, a wavelength conversion device and wavelength conversion method capable of efficiently converting incident light to light of a converted wavelength without lowering the light output of the converted wavelength.
Furthermore, another object is to obtain a BBO crystal for wavelength conversion to simplify resonator length conversion used for the conversion of incident light to light of a converted wavelength.